I. Field of the Invention
This invention relates generally to electrical switch panels and more particularly to the novel design of a switch panel in which a positive indication is given to the operator that a switch contact has been established. Panels of the type involved herein find wide application in a variety of electrical devices including hand calculators, typewriter type keyboards, and computer alpha-numeric input panels.
II. Description of the Prior Art
Diaphragm-type switches in which a force is applied to a membrane to thereby deform it through an aperture in a spacer and establish electrical contact with a pattern of conductors disposed at the bottom of the aperture have long been used and are well known in the art. Typical of these prior art diaphragm-type touch panels is that described in the Comstock U.S. Pat. No. 3,591,749 and the IBM Technical Disclosure Bulletin, Vol. 14, No. 3 dated August 1971 and entitled "Elastic Diaphragm Switch" by L. H. Sedaris and K. B. Stevens. Still another prior art patent is U.S. Pat. No. 3,600,528 to Leposavic. The switch panels made in accordance with the aforementioned IBM Technical Disclosure Bulletin and the Leposavic patent are not altogether satisfactory in that they do not provide a positive indication to the operator by way of tactile feedback to indicate that a switch closure has been accomplished. Various attempts have been made to achieve tactile feedback through the use of the so-called "oil-can effect" wherein the diaphragm is bowed slightly upward and when depressed through its median line provides a snap feel and possibly an audible click, the diaphragm returning to its original position when the finger force is removed. One such arrangement is disclosed in the Lynn et al. U.S. Pat. No. 3,860,771 which discloses the use of a dome-shaped projection integrally formed with a cylindrical pedestal, there being a conductive material on the underside of the dome. The dome inverts through the pedestal and through an underlying spacer to bridge contacts on a hard board when force is applied. When the force is released, the oil-can effect restores the dome to its normal arched, non-contacting position.
Devices made in accordance with certain aspects of the prior art suffer from a defect which may be termed "edge toggle". Edge toggle occurs when only one portion of the dome collapses to produce tactile feedback, or when one portion of the dome collapses late and produces a double tactile feedback sensation. This edge toggle always occurs along a crease line where the slope of the crease's center wall approaches the vertical. Five characteristics of edge toggling may be observed and are as follows:
1. the action is not concentric and proceeds from the center of the dome to only one segment of the outer circumference thereof;
2. the collapse of the dome is not catastrophic and does not always go to completion;
3. movement of the flexible dome material is not always isolated within the dome and tends to lift the surrounding circuit;
4. the tactile feedback sensation is very dependent upon the location on the dome where the force is applied;
5. the tactile feedback is not constant and may be different every time the dome is collapsed.
It is a principal object of the present invention to provide a switch panel comprising a plurality of diaphragm-type switches in which the problem of edge toggling is obviated.